The present invention relates to a pedestal control circuit used in a broadcasting color TV camera or the like and, more particularly, a pedestal control circuit wherein the pedestal level of the gamma-corrected video signal does not change even if the gamma correction characteristic is changed for color adjustment.
In high-quality color TV monitors for broadcasting or the like, pedestal control (pedestal correction) and gamma correction are performed as important functions of color adjustment for optimal color reproduction and contrast. Pedestal control is performed so as to obtain an optimal black level of color signals and to match the black levels of the red, green and blue color signals. Gamma correction is performed so as to achieve correct color reproduction within a full range of luminance information from black to white by compensating non-linearity of the brightness vs. cathode current characteristic of a CRT using a complementary inverted characteristic.
In a conventional circuit configuration, pedestal correction (or DC restoration) is followed by gamma correction. However, with this circuit configuration, the pedestal level fluctuates each time the gamma correction characteristic is changed. For example, referring to FIG. 3, when video signal S22A is an input signal to a gamma corrector to be gamma-corrected, the gamma corrector produces signal S16A as gamma-corrected output in accordance with gamma correction characteristic curve A. When gamma correction curve A is changed to B, a gamma-corrected output for input S22A becomes signal S16X. As can be seen from a comparison of pedestal level eplA of signal S16A and pedestal level eplX of signal S16X, in the conventional circuit configuration, a change in gamma correction characteristic (e.g., curve A to B) results in fluctuation (eplA to eplX) in pedestal level of the gamma-corrected output (S16). For this reason, the pedestal level of the gamma-corrected output must be re-adjusted each time gamma correction adjustment is performed. If this re-adjustment is not performed, correct black level reproduction cannot be performed.
The above is a problem encountered in gamma correction in conventional pedestal control circuits.
When the pedestal levels of red (R), green (G) and blue (B) are controlled by a single pedestal control voltage, the pedestal levels of the R, G and B color signals vary unless the gamma correction characteristics of the respective color signals completely coincide with each other. When the pedestal levels of the R, G and B color signals vary, correct black level reproduction cannot be performed. Assume that the R signal gamma correction curve is curve A in FIG. 3 and the G and B signal gamma correction curves are curve B in FIG. 3. Under this assumption, when pedestal correction of the G and B color signals is performed, the pedestal level of the R color signal is deviated from the optimal value. In order to solve this problem, instead of adjustment by a single pedestal control voltage, pedestal correction adjustment must be independently performed for each of the R, G and B color signals. However, this results in a complex adjustment process in the actual manufacture and causes variations in the pedestal correction adjustment.
The above problem is presented when pedestal correction of two or more color signals is performed with a conventional pedestal control circuit.